VALES VI: ISM enrichment in star-forming galaxies up to z∼0.2 using ^12CO(1-0), ^13CO(1-0) and C^18O(1-0) line luminosity ratios

We present Atacama Large Millimeter/sub-millimeter Array (ALMA) observations towards 27 low-redshift (0.02< z<0.2) star-forming galaxies taken from the Valparaíso ALMA/APEX Line Emission Survey (VALES). We perform stacking analyses of the ^12CO(1-0), ^13CO(1-0) and C^18O(1-0) emission lines to explore the L' (^12CO(1-0))/L'(^13CO(1-0))) (hereafter L'(^12CO)/L'(^13CO)) and L'(^13CO(1-0))/L'(C^18O(1-0)) (hereafter L'(^13CO)/L'(C^18O) line luminosity ratio dependence as a function of different global galaxy parameters related to the star formation activity. The sample has far-IR luminosities 10^10.1-11.9L_⊙ and stellar masses of 10^9.8-10.9M_⊙ corresponding to typical star-forming and starburst galaxies at these redshifts. On average we find a L'(^12CO)/L'(^13CO) line luminosity ratio value of 16.1±2.5. Galaxies with evidences of possible merging activity tend to show higher L'(^12CO)/L'(^13CO) ratios by a factor of two, while variations of this order are also found in galaxy samples with higher star formation rates or star formation efficiencies. We also find an average L'(^13CO)/L'(C^18O) line luminosity ratio of 2.5±0.6, which is in good agreement with those previously reported for starburst galaxies. We find that galaxy samples with high L_IR, SFR and SFE show low L'(^13CO)/L'(C^18O) line luminosity ratios with high L'(^12CO)/L'(^13CO) line luminosity ratios, suggesting that these trends are produced by selective enrichment of massive stars in young starbursts.